A rubber material having a viscous elasticity follows a hysteresis behavior and thus a tread portion of a tire generates a heat by repeating deformation and contraction due to rolling. If the rubber material constituting the tread portion increases, a hysteresis loss due to a bending deformation or a shear deformation at the time of tire rolling increases. Thus, a tire which is large in thickness of the tread portion is likely to increase in temperature.
In particular, a large-scaled tire employed in a large-scaled vehicle used in mine or construction site or the like is characterized in that the tire is not only large in amount of rubber material used, and but also is used in a heavy loading state, on a rough road surface, and under a severe traction condition, and the tire repeats deformation and contraction, and is thus likely to generate a heat. If the tire reaches a high temperature while in cruising, it causes delamination (separation) between the rubber material forming the tread portion and a belt layer or the like, thus leading to acceleration of an exchange cycle of the tire.
Incidentally, there is conventionally known a method of reducing an amount of rubber material that is a heat generation source by forming a subsidiary groove along a tread widthwise direction at a tread portion, and promoting heat radiation of the tread portion by increasing a surface area of the tread portion (for example, Patent Literature 1).